Did you know that goldfish have no stomachs that is why they will eat and eat if you feed them often, I have just recently been told this by a fish expert 'Well I never'
While goldfish do pull the nutrients in from the food you feed them, they dont have a digestive system. I wish this expert would have told you that rather than saying he had no stomach because indeed they do, just not like a humans.
Cheers I found the information below on the internet so it is almost the same as what I was told, but I also read that their stomach is only slightly larger than their eye so which is it, no stomach or small stomach?
Koi do not have stomachs. All their food is digested in the intestinal track so they never fill up. As food is going in one end it is pretty much coming out the other end. So you have to control how much food they eat. Not only that if you over feed you end up with fat fish and that is not good. And the more you feed the dirtier the water will become.